FIG. 1 is a schematic view of a conventional ultrasonic flowmeter including ring-shaped ultrasonic transducers 1a and 1b, respectively acting as a transmitter and a receiver, provided around a tube 2 as being away from each other by a predetermined interval (Japanese Laid-Open Patent Publication No. Hei 10-122923). The ultrasonic flowmeter uses a total of two ring-shaped ultrasonic transducer, one as a transmitter and the other as a receiver. With such a structure, the level of an ultrasonic signal is weak and it is difficult to perform measurement. With a structure in which the ultrasonic transducers are directly attached to the tube having a fluid flowing therein, reverberation that occurs due to the long ultrasonic signal influences the next ultrasonic signal that is to be transmitted or received. For this reason, the measurement cannot be performed stably.
With the above-described ultrasonic flowmeter, the ultrasonic transducers acting as a transmitter and a receiver need to be formed so as to be suited to the diameter of the tube. In addition, the tube needs to be cut when the ultrasonic transducers are attached. In general, many ultrasonic flowmeters have a structure suitable to be installed between pipes. In the case where an ultrasonic flowmeter is installed to a pipe already installed, a work of, for example, cutting the pipe is needed in order to install the ultrasonic flowmeter. For this reason, it is difficult to install the ultrasonic flowmeter to a pipe already installed.
According to an existing method for installing a flowmeter without cutting a pipe, a semicircular, annular ultrasonic transducer obtained as a result of a ring-shaped ultrasonic transducer being cut into two is installed. However, this method is not practical for the following reasons. The semicircular, annular ultrasonic transducer is formed of a ceramic material. Such an ultrasonic transducer is very difficult to be precisely produced so as to be in close contact with a tube, and production thereof costs high.